The effect of TCP morphology on the development of aluminide coated superalloys

Aluminisation gives a cost-effective coating for turbine components subjected to hostile environments. However, inter-diffusion between coating and substrate causes changes in microstructure, which destroy the creep resistance of the alloy. We compare low-temperature, high-activity aluminide and platinum-modified aluminide coatings on the single crystal, nickel-based alloys, RR3000, CMSX-4 and TMS75, and describe the evolution of the microstructure following exposure at 1100 °C. In particular, the precipitation of topologically close packed (TCP) phases varies with alloy composition. We suggest that the morphology of the precipitates may influence the rate of ingress of the coating into the substrate. For example, the alloy RR3000 forms the σ-phase, which has an excellent fit with the alloy {1 1 1} planes and can nucleate as very fine small precipitates. In contrast, the μ-phase formed in the alloy CMSX-4 forms much larger, more widely spaced, TCP phases which necessitate a longer diffusion path for the slow diffusing refractory elements of which they are principally composed.